This application is related to Japanese Patent Application No. 11-189132, filed on Jul. 2, 1999, the entire contents of which are incorporated.
1. Field of the Invention
The present invention relates to a printing system that enables adjustment of positional misalignment of dot creation, an equivalent method of adjustment, and a recording medium.
2. Discussion of the Background
Ink jet printers that cause ink to be ejected from a print head and thereby implement printing have been widely used as an output device of the computer. The ink jet printer moves the print head back and forth relative to a printing medium during a main scan and ejects multiple color inks to create dots. Variable dot printers that are capable of expressing multiple densities with regard to each pixel, instead of just two stages, dot-on and dot-off, have recently been proposed. The multiple densities are attained, for example, by creating dots of different ink quantities.
In some of these ink jet printers, dots are created during both forward and backward movements in the main scanning direction for the purpose of enhancing the recording speed (hereinafter this recording process is referred to as bidirectional recording). In this case, it is required to make the positions of dots created in the forward pass coincident with the positions of dots created in the backward pass in the main scanning direction. A relative positional misalignment between dots created in the forward pass and dots created in the backward pass is attributable to a rough touch to a resulting printed image and deteriorates the picture quality of the resulting printed image. One proposed method to reduce such a positional misalignment utilizes a predetermined test pattern for the adjustment. The background art technique adjusts the creation timings of, for example, black dots in the forward pass and in the backward pass to reduce the positional misalignment of dots in the course of bidirectional recording.
This background art adjustment technique can not, however, sufficiently reduce the positional misalignment of dots in the variable dot printer that creates dots of different ink quantities. The flight characteristics of an ink droplet ejected from the print head depend upon the ink quantity. In some cases, even when the dot creation timing is adjusted for one of the variable dots, the adjustment of the dot creation timing may be insufficient for the other dots. The background art adjustment technique leaves some dots having an insufficiently corrected positional misalignment in the course of bidirectional recording. This causes deterioration of the picture quality in the case of bidirectional recording.
In the case of bidirectional recording, even a slight positional misalignment of dot creation often significantly affects the picture quality. For example, it is assumed that a print head moves left and right during a main scan and creates dots in the forward pass at positions deviated leftward from the expected positions. Because of the characteristic of the print head, dots are created in the backward pass at positions deviated rightward from the expected positions. The relative positional misalignment between the dots created in the forward pass and the dots created in the backward pass in the course of bidirectional recording is approximately twice the positional misalignment of dots created in only one of the forward pass and the backward pass. Namely, the presence of dots having insufficiently adjusted creating positions significantly deteriorates the picture quality in the case of bidirectional recording.
The printer is generally required to perform high-quality and high-speed printing. Bidirectional recording with a printing speed almost double that of unidirectional recording is desirable for the purpose of the high-speed printing. Due to the positional misalignment of dots discussed above, bidirectional recording produces a reduced picture quality and is thus used in a print mode that gives preference to printing speed over picture quality. With the recent advance of the printer for the higher resolution and a higher picture quality, an image tends to be printed by a greater number of passes of the main scan. Since unidirectional recording has a low printing speed, the enhanced printing speed of the bidirectional recording is highly demanded. The recent trend simultaneously requires an extremely high picture quality, even in the case of bidirectional recording. In the variable dot printer that enables a multi-value expression in each pixel for the improved picture quality, the deterioration of the picture quality due to the positional misalignment of dots in the course of bidirectional recording is of great significance.
The variable dot printer varies the quantity of ink ejected from an identical nozzle and enables dots of different ink quantities to be created in respective pixels. No technique has been proposed to adjust the positional misalignment of variable dots created by the identical nozzle. Further, different ink quantities generally result in different flight velocities of the ink droplets. There is accordingly a positional misalignment of dots having different ink quantities even when they are created by the identical nozzle.
The ink ejection timing is set in advance to prevent the positional misalignment by taking into account the differences in flight velocity. However, it is extremely difficult to completely cancel such a positional misalignment, since there is a variation in flight velocity of the ink droplet due to the variation in manufacturing error of the print head. The positional misalignment of dots is also ascribed to a variation in thickness of printing paper. In the case of thick printing paper, there is a smaller distance between the print head and the printing paper. This shortens the flight time of the ink droplet. In the case of thin printing paper, on the contrary, the ink droplet has a longer flight time. The ejection timings to form the ink droplets of different flight velocities at an identical position are set, based on the relation to the flight time. When the thickness of the printing paper is changed from the initial setting, there is a positional misalignment of ink droplets. Such a positional misalignment significantly damages the picture quality even in the case of creating dots only in one direction of the main scan.
Accordingly, one object of the present invention is to solve the above-noted and other problems.
Another object of the present invention is to provide a technique that reduces a positional misalignment of dots in the course of bidirectional recording in a printing system that enables variable expression in each pixel.
At least part of the above and the other related objects is attained by a printing system that creates dots on a printing medium with a print head in the course of main scan and thereby prints an image. The print head is capable of creating n variable dots (where n is an integer of not less than 2), which at least partly include dots of an identical ink but different ink quantities, in response to driving signals. The printing system also includes a memory that stores output timings of n driving signals corresponding to the n variable dots, a driving signal output unit that outputs at least part of the n driving signals to the print head in the course of the main scan, according to dots to be created in respective pixels, and a timing adjustment unit that individually adjusts the output timing stored in the memory with regard to each of the n variable dots.
The printing system of the present invention enables the output timings of the driving signals to be adjusted with regard to each of the n variable dots creatable by the print head. This arrangement effectively reduces the positional misalignment of the respective dots and thereby improves the picture quality of the printed image. The n variable dots include dots of an identical ink but different ink quantities.
A printer using dots of different ink quantities has recently been developed. There has been substantially no discussion on the necessity of adjusting the position of dot creation according to the quantity of ink. In general, dot forming elements provided on the print head are commonly used to create dots of an identical ink but different ink quantities. There has also been substantially no discussion on the necessity of adjusting the output timings of the driving signals for each ink quantity. The inventors of the present invention have found that the commonly used dot forming elements may cause a positional misalignment of dots having different ink quantities and that such a positional misalignment of dots significantly deteriorates the picture quality. The inventors have also noticed that adjustment of the output timings of the driving signals for each ink quantity with regard to the dot forming elements commonly used to create a plurality of variable dots significantly improves the picture quality.
Further, the term xe2x80x98adjustmentxe2x80x99 in the specification represents a process of varying the output timings in response to a predetermined external operation or in response to an input of an instruction while the printing system is in a working state. It is not necessary that all the n variable dots have different ink quantities. For example, in a printing system which creates dots with multiple color inks or with inks of different densities, the n variable dots may include dots of an identical ink quantity, but different hues or different densities. The requirement is that all the n variable dots do not have an identical ink quantity.
In accordance with one preferable application of the printing system according to the present invention, the memory stores the output timings of the n driving signals with regard to a forward pass of the main scan separately from those with regard to a backward pass of the main scan. The driving signal output unit outputs at least part of the n driving signals to the print head in each of the forward pass and the backward pass of the main scan. Further, the timing adjustment unit individually adjusts the output timing stored in the memory with regard to each of the n variable dots, so as to allow a relative change of the output timing in the backward pass to the output timing in the forward pass.
The printing system of such an arrangement enables the output timings of the driving signals in the backward pass of the main scan and the output timings in the forward pass to be regulated relative to each other with regard to each of the n variable dots creatable by the print head. Either one of the output timings in the forward pass and the output timings in the backward pass may be regulated, or the output timings in both the forward pass the backward pass may be regulated individually. Regulating the dot creation timings in the forward pass and the backward pass relative to each other effectively reduces the positional misalignment arising in the course of bidirectional recording with regard to each of the variable dots, thereby significantly improving the picture quality of the resulting printed image.
Further, a diversity of configurations may be applied for the adjustment of the output timing of the driving signal according to the structure of the print head that is capable of the n variable dots.
In accordance with one preferable embodiment, the printing system of the present invention further includes: a reference signal output unit that outputs a reference signal, which is related to the output of the driving signal, at a fixed period corresponding to each pixel, based on a rate of the main scan. In this embodiment, the print head drives specific elements that correspond to the n variable dots, in order to create the n variable dots, and the memory stores therein a delay time from the reference signal as each of the output timings.
The printing system of such arrangement has the specific elements that correspond to the n variable dots and are provided on the print head. Namely the driving signal output to each pixel is one-to-one mapped to the type of dot created in the pixel. The arrangement of storing the delay time individually for each of the driving signals enables adjustment of the output timing. It is not necessary to provide n different elements, but a plurality of specific elements may be driven simultaneously to attain the n different states.
In accordance with another embodiment of the printing system, the driving signal output unit includes: an original driving signal output unit that successively outputs a plurality of original driving signals to each pixel; and a selection unit that selects at least part of the original driving signals, so as to generate the n driving signals corresponding to the n variable dots. In this embodiment, the memory stores therein an interval of each original driving signal as each of the output timings.
The printing system of such arrangement selects the on-off state of the plurality of original driving signals output to each pixel, so as to create the n variable dots. The simplest configuration successively outputs n different original driving signals, which respectively correspond to the n variable dots, to each pixel and selects one of the n different original driving signals to create the corresponding dot. In this application, the position of the dot created in each pixel is adjusted by regulating the interval of the selected original driving signal. In this structure, the desired dot may be created with a plurality of the original driving signals allocated to each pixel. It is not necessary to output the n different original driving signals to each pixel, but the requirement is that the n different driving signals are actualized by the combinations of the on-off state of the plural original driving signals.
In accordance with still another preferable embodiment, the printing system of the present invention further includes a test pattern printing unit that prints a predetermined test pattern, which is set to enable detection of a relative positional misalignment of dots created under different printing conditions, with regard to each of the n variable dots.
This arrangement enables the user of the printing system to relatively easily adjust the positional misalignment of dots. Here the different printing conditions include, for example, different directions of the main scan to create dots, different print heads used to create dots, and different driving waveforms used to create dots.
In the application that performs the adjustment based on the test pattern, it is preferable that the timing adjustment unit adjusts the output timing, based on relation to the printed result of the predetermined test pattern.
One concrete procedure prints the test patterns with indexes representing the respective output timings and selects the index to specify the desired output timing that gives the optimum printed result. This further facilitates the adjustment.
The variable dot printer generally uses a large number of variable dots, which are the potential target of adjustment of the positional misalignment. It is of course possible to successively adjust the output timing for all these variable dots. But this method takes an extremely long time for the adjustment and requires significantly burdensome work.
In one preferable embodiment of the application that adjusts the output timing based on the test pattern, the printing further includes: an inter-dot comparison pattern printing unit that prints predetermined patterns, in each of which dots created under different printing conditions are mixed, with regard to at least two variable dots of interest creatable by the print head, in a specific arrangement that enables mutual comparison; and a selecting instruction input unit that inputs a selecting instruction to select a specific dot among the at least two variable dots of interest, which are subjected to the printing by the inter-dot comparison pattern printing unit. In this embodiment, the test pattern printing unit prints the predetermined test pattern with regard to the selected specific dot.
This arrangement enables the user to compare the predetermined patterns printed by the inter-dot comparison pattern printing unit with each other and select the dot having a significant positional misalignment arising in the course of bidirectional recording. The adjustment based on the test pattern is performed only for the selected specific dot. This arrangement significantly alleviates the burden of the adjustment.
A diversity of conditions can be set to the different printing conditions in the inter-dot comparison pattern printing unit according to the dots of interest specified as the target of adjustment. For example, in the case where the n variable dots of an identical color but different ink quantities are specified as the target of adjustment, the predetermined pattern includes the dots created in the different directions of the main scan. When the three variable size dots, that is, the large-sized dot, the medium-sized dot, and the small-sized dot, are specified as the target of adjustment, three patterns are printed; the pattern in which dots created in the forward pass are mixed with dots created in the backward pass with regard to the large-sized dot, the pattern in which dots created in the forward pass are mixed with dots created in the backward pass with regard to the medium-sized dot, and the pattern in which dots created in the forward pass are mixed with dots created in the backward pass with regard to the small-sized dot. The user selects the pattern having the most significant rough touch among these three.
In the case where the dots of different colors are specified as the target of adjustment, the predetermined pattern includes the dots created with different inks. When the dots of cyan, magenta, and yellow are specified as the target of adjustment, two patterns are printed; the pattern in which dots of cyan and magenta are mixed and the pattern in which dots of cyan and yellow are mixed. The user selects the pattern having the more significant rough touch between these two and carries out the adjustment of the output timing with regard to the selected color. This arrangement enables the positional misalignment of dots to be adjusted using cyan as the reference color. In this example, magenta or yellow may alternatively be set as the reference color.
In the printing system of the present invention, the output timing of the driving signal corresponding to each of the variable dots is adjusted on the basis of the ideal timing that is specified by the relation to the pixel as the absolute criterion.
The timing adjustment unit may adjust the output timing of another driving signal corresponding to another dot, relative to the output timing of one driving signal corresponding to one reference dot selected among the n variable dots. Namely this unit adjusts the output timing of another driving signal relative to the dot creation timing of the reference dot.
In the application that relatively adjusts the output timing, the reference dot may be any of the n variable dots.
In one desirable embodiment, the reference dot has a substantially intermediate ink ejection speed among the n variable dots.
In another desirable embodiment, the reference dot has a substantially intermediate output timing of the corresponding reference signal among the n variable dots.
The former application selects the dot having the substantially intermediate ink ejection speed, that is, the ink ejection speed close to the xe2x80x98(maximum ink ejection speed+minimum ink ejection speed)/2xe2x80x99 as the reference dot when the n variable dots have different ink ejection speeds. The latter application pays attention to the previous output timings and selects the dot having the substantially intermediate output timing, that is, the output timing close to the xe2x80x98(earliest output timing+latest output timing)/2xe2x80x99 as the reference dot.
Selecting the reference dot in this manner advantageously enables the output timings of the driving signals with regard to the other dots to be readily adjusted. For example, in the case where the dot having the extremely early output timing is set as the reference dot, there is a possibility that the driving signal is to be output prior to supply of print data corresponding to each pixel, with regard to the dot that requires the earlier output timing than that of the reference dot. In the case where the dot having the extremely late output timing is set as the reference dot, on the contrary, there is a possibility that the driving signal is to continuously output even after the time point when print data corresponding to a next pixel is supplied, with regard to the dot that requires the later output timing than that of the reference dot. In either case, appropriate printing can not be attained. The selection of the dot having the substantially intermediate output timing as the reference dot ensures the appropriate adjustment of the output timing without causing such problems. The above description is also applicable for the ink ejection speed.
In the above applications, the substantially intermediate ink ejection speed and the substantially intermediate output timing are just the standards for selecting the dot that enables adequate adjustment of the output timing as the reference dot. It is not necessary that the ink ejection speed or the output timing of the reference dot is strictly identical with its intermediate value. It is also not necessary to select the dot having the substantially intermediate value as the reference dot.
The technique of the present invention is also attained by a method of adjusting the positional misalignment of dots as discussed below, other than the printing system.
The present invention is thus directed to a method of individually adjusting an output timing of each driving signal corresponding to each of n variable dots in order to correct a relative positional misalignment of dots, in a printing system that creates dots on a printing medium with a print head in the course of main scan and thereby prints an image. Here the print head is capable of creating n variable dots (where n is an integer of not less than 2), which at least partly include dots of an identical ink but different ink quantities, in response to driving signals. The method includes the steps of: (a) printing a predetermined test pattern, which is set to enable detection of a relative positional misalignment of dots created under different printing conditions, with regard to each of the n variable dots; (b) inputting a preset output timing of a selected driving signal, which is specified, based on a relation to the printed test pattern; and (c) changing an existing value of the output timing to the input output timing.
Like the printing system that adjusts the positional misalignment of dots based on the printed test pattern, this method enables the positional misalignment of dots to be adjusted relatively easily. This arrangement allows the adjustment of the positional misalignment with regard to each of the n variable dots and thereby significantly improves the picture quality of the resulting printed image.
In accordance with one preferable application of the present invention, the method further includes the following steps, prior to the step (a): (a1) printing predetermined patterns, in each of which dots created under different printing conditions are mixed, with regard to at least two variable dots of interest creatable by the print head, in a specific arrangement that enables mutual comparison; and (a2) inputting a selecting instruction to select a specific dot among the at least two variable dots of interest, which are subjected to the printing in the step (a1). In this application, the step (a) prints the predetermined test pattern with regard to the specific dot selected in the step (a2).
As discussed previously on the printing system, when there are a large number of dots, which are the potential target of adjustment, this arrangement effectively alleviates the burden of the adjustment. As in the case of the printing system, a variety of conditions according to the dots of interest specified as the target of adjustment may be set to the different printing conditions.
When the print head is capable of creating dots with multiple color inks, it is preferable that the method includes the steps of: (A) carrying out the steps (a) through (c) with regard to dots created under different printing conditions with one identical ink selected among the multiple color inks; and (B) carrying out the steps (a) through (c) with regard to dots created with different inks selected among the multiple color inks.
This arrangement enables adjustment of the positional misalignment, which occurs in the course of bidirectional recording, between variable size dots having different ink quantities as well as between different color inks. The steps (A) and (B) may be carried out in a variety of combinations. One applicable procedure adjusts the positional misalignment between variable size dots having different ink quantities with regard to each color ink in the step (A) and subsequently adjusts the positional misalignment between different color inks in the step (B). Another applicable procedure adjusts the positional misalignment between variable size dots with regard to one selected reference color in the step (A), subsequently adjusts the positional misalignment between different color inks in the step (B), and again adjusts the positional misalignment between variable size dots with regard to the other color inks but the reference color in the step (A). In the method of this arrangement, either of the steps (A) and (B) may accompany the steps (a1) and (a2) discussed above.
The technique of the present invention is further attained by a recording medium discussed below.
The present invention is accordingly directed to a recording medium in which a program is recorded in a computer readable manner to individually adjust an output timing of each driving signal corresponding to each of n variable dots and thereby correct a relative positional misalignment of dots in a printing system that creates dots on a printing medium with a print head in the course of main scan and thereby prints an image. Here the print head is capable of creating the n variable dots (where n is an integer of not less than 2), which at least partly include dots of an identical ink but different ink quantities, in response to driving signals. The program causes a computer to attain: a test pattern printing function that prints a predetermined test pattern, which is set to enable detection of a relative positional misalignment of dots created under different printing conditions, with regard to each of the n variable dots; a function of inputting a preset output timing of a selected driving signal, which is specified, based on a relation to the printed test pattern; and a function of changing an existing value of the output timing to the input output timing.
In accordance with one preferable application, the program further causes the computer to attain: an inter-dot comparison pattern printing function that prints predetermined patterns, in each of which dots created under different printing conditions are mixed, with regard to at least two variable dots of interest creatable by the print head, in a specific arrangement that enables mutual comparison; and a function of inputting a selecting instruction to select a specific dot among the at least two variable dots of interest, which are subjected to the printing by the inter-dot comparison pattern printing function. Here the test pattern printing function prints the predetermined test pattern with regard to the selected specific dot.
Execution of the above program attains the printing system and the method discussed above. Typical examples of the recording medium include flexible disks, CD-ROMs, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. The present invention may also be directed to a program itself for attaining the functions discussed above and a variety of signals equivalent thereto.